A. Field of the Invention
The present invention relates to hard disk drives. More specifically, the present invention relates to improving dynamic properties of a gimbal in a hard disk drive.
B. Description of Related Art
In hard disk drives (HDD), reading and writing of the information on a disk is accomplished by a transducer on a slider mounted on a suspension. The suspension itself can have a number of components, including a load beam, a gimbal, circuit traces, hinges and a base plate. The suspension supports the transducer mechanically, and connects the transducer and a pre-amplifier electrically.
FIG. 1A illustrates a typical HDD 100, including at least one disk 110, which a spindle motor can cause to rotate around the axle of a central drive hub 120. A read/write transducer or head 130 is attached to a load beam 140 of a suspension via a slider and a gimbal, as shown in more detail in FIG. 1B. The load beam 140 is supported by an actuator arm 150 of an actuator 160. In operation, the disk 110 rotates at high speed, creating an air bearing which causes the head 130 to float over the disk surface. The actuator 160 moves the head 130 across the tracks on the disk until the head is positioned at a target track.
FIG. 1B illustrates the actuator arm 150 in more detail. As shown, the read/write head 130 is mounted on a slider 142. The slider 142 is coupled to the load beam 140 through a gimbal 143. The head 130 is held in a close relative position over data tracks formed on the disk 110 to permit a read or write operation. Because of the flexibility of the gimbal 143, the slider 142 can remain flexible while floating over the disk 110.
The high speed rotation of the disk 110 generates air flow, which cooperates with an air bearing surface (ABS) of the slider 142 to enable the slider to fly above the disk 110. The slider 142 should fly as close to the surface of the disk 110 as possible without contacting the disk. The spacing between the disk and the slider is known as the “flying height,” and may be about 10 nm in some HDD.
FIGS. 2A, 2B, 2C and 2D illustrate known gimbals in more detail. As shown, the gimbal 143 has three layers: a stainless steel layer 210, a polymer layer 220, and copper circuit traces 230 on the polymer layer. The gimbal 143 is attached to the load beam 140. As shown in FIG. 2C, a dimple 240 may protrude from the back of the load beam and contact a flexure tongue on the stainless steel layer 210 of the gimbal. The center of the dimple 240 is at the center of an XYZ coordinate shown in FIG. 2A. The slider 142 is attached to the gimbal 143 via an epoxy layer 250.
In order to make the slider 142 fly stably and reliably with a small gap between the slider and the disk, suspension design has included a number of requirements, such as vertical stiffness (Kz), gimbal pitch and roll stiffness (Kp, Kr), gimbal static attitude—including pitch and roll static attitude (PSA/RSA), among others. These requirements are mainly static and geometrically based. Another important requirement is that the suspension has no or little dynamic effect on the slider air bearing.
A dynamic effect could be the result of head-disk contact. Another cause of a dynamic effect could be operation at high altitudes, where the air is thinner because there is less air under the slider, causing the flying height to be lower. Under these circumstances, the suspension and the slider both can be set into motion, causing dynamic effects.
Suspension dynamic performance is important in some situations. For instance, it is an important factor affecting what may be called the take-off altitude of a slider in a hard disk drive. For example, to pass a qualification test, a slider must be able to fly over a disk normally to access data on the disk when the HDD is not higher than 10,000 feet (which would mean that the take-off altitude of the HDD is 10,000 feet). At higher altitudes, the slider flies with a lower flying height and/or lower pitch angle. If the slider is disturbed by a shock or a particle, it can contact the disk and vibrate. Below the take-off altitude, the vibration of the slider will dampen, and the slider will settle down and resume normal operation. Above the take-off altitude, the slider will keep bouncing vibration, potentially preventing take off and causing an HDD failure.
The bouncing vibration (resulting from the head hitting the disk surface) is a self-excited vibration. Sufficient damping weakens vibration. If the energy that the slider obtains from the disk is greater than the energy loss due to damping, the vibration will not dampen sufficiently and the drive will fail. A high damping can prevent a sustained vibration. The damping of the suspension comes mainly from material, the friction between the gimbal and the load beam, and the friction between the dimple and the flexure tongue.
The amount of friction depends on the mode shapes of the suspension and the gimbal. Current suspension gimbals have symmetric dynamic properties. Although the copper traces of the gimbal shown in FIG. 2B appear not to be symmetric, the mode shapes of the gimbal are symmetric about the axis of the gimbal, as shown in FIG. 2D, and the dynamic properties of the gimbal are symmetric. This is because the main structure of the gimbal—the stainless steel layer of the gimbal shown in FIG. 2B—is symmetric. A symmetric suspension and gimbal will tend to have smaller amounts of friction, and thus will tend to provide lower damping. With such structure, there is a good chance that when a vibration occurs, it will not dampen, even at a relatively low altitude. A hard disk drive with the gimbal shown in FIG. 2A has a sustained vibration at 54 kHz, which makes the slider take off at 9500 feet, lower than the 10,000 feet required by the qualification test.
It is therefore desirable to have a gimbal, suspension and hard disk drive with improved dynamic properties.